There have been image-input type display units which can serve both as an image input terminal and an image display each by combining a liquid-crystal display (LCD) panel with a two-dimensional image sensor and which are, for example, disclosed in Japanese Laid-open Patent Publications No. 4-282609 (TOKKAI HEI) and No. 6-186585 (TOKKAI HEI). The display unit described in Japanese Laid-open Patent Publication (TOKKAI HEI) No. 4-282609 comprises a color liquid-crystal display panel, a two-dimensional image sensor mounted on the reverse surface of the color LCD-panel and a light-source. When an original placed on the top surface of the color LCD-panel is illuminated by light emitted from the light source trough the LCD-panel, light reflected from the original through again the LCD-panel is received by the image sensor for reading an image of the original.
The display unit disclosed in Japanese Laid-open Patent Publication (TOKKAI HEI) No. 6-186585 is composed of a LCD panel, an image sensor composed of a photo-diodes formed at grid points of a non-display portion of the LCD-panel and a light-source. This display unit in a similar way as described above reads an image of an original placed on a top surface of the color LCD-panel by illuminating with light emitted from the light source through the panel and by receiving light reflected from the original by a photo-diode.
In the above-mentioned device, the image sensor is formed on another surface than a surface of the color LCD panel where a liquid-crystal driving electrode or a TFT (thin film transistor). In other words, elements different in function are merely laid on each other.
On the other hand, Japanese Laid-open Patent Publication (TOKKAI HEI) No. 4-251824 discloses another display unit wherein a LCD panel and an image sensor are combined with each other more effectively.
This device is constructed of an upper substrate with a non-linear resistive element formed thereon, a lower substrate, liquid-crystals held between the upper substrate and the lower substrate, a polarizer stuck to a top surface of the upper substrate, a polarizer stuck to a bottom surface of the lower substrate, a light-source for a back light and a light-guide plate for leading the light from the light source into the liquid-crystal layer.
The non-linear resistive element sandwiched between a pixel electrode and a signal electrode is in contact with the signal electrode. The lower substrate has a scanning electrode through which it is in contact with the liquid crystal.
The above-mentioned device construction is the same as that of a two-terminal matrix type LCD panel which is represented by a MIM (metal-insulator-metal) diode excepting the former adopting non-linear resistive elements having light-sensitivity.
The optical writing operation of the device is as follows:
The liquid crystal is put into a transparent state and a desired voltage is applied across the pixel electrode and the scanning electrode, then an original is illuminated. Light reflected from the original falls on non-linear resistive elements, those of which received light reflected from white portions of the original reduce its resistance, causing reduction of an effective voltage applied to the liquid crystal molecules which in this case are aligned parallel to the substrates. As light is absorbed by black parts of the original, no light falls on corresponding non-linear resistive elements which have no change in its resistance and therefore keep the liquid crystals as be aligned perpendicular to the substrates. Thus, an image of the original is written directly into the liquid crystal display panel by light.
The electrical image-reading method is as follows:
The non-linear resistive elements are classified into resistive components and capacitive components, which are connected in parallel to each other. The liquid crystals are also classified into resistive components and capacitive components, which are connected in parallel to each other. The non-linear resistive elements are connected in series to the liquid crystals. When reflected light rays from the original image fall on non-linear resistive elements and liquid crystals, resistive components of the resistive elements and capacitive components of the liquid crystals decrease. With a voltage being applied across the pixel electrodes and scanning electrodes, current flowing illuminated pixels may differ in value from current flowing not-illuminated pixels. The written image can be read by detecting differential currents of pixels.
As described above, both the prior art display units provided with an image input facility, which are disclosed in Japanese Laid-open Patent Publications No. 4-282609 (TOKKAI HEI) and No. 6-186585 (TOKKAI HEI), are manufactured each by merely laminating an image sensor on a liquid-crystal display panel. Each device is complicated in structure because of forming a LCD layer (display electrodes) and an image inputting sensor layer (image-inputting electrodes) on different surfaces.
To read an original image and display it on a LCD panel, each device has to convert the optical image into electrical signals through processes of reading the original image by the image sensor, converting electrical signals of the read image into electrical signals for display, displaying the image by electrical signals on the display panel. The device may have errors in the processes resulting in lowering the displayed image quality.
An image displayed on the LCD panel can be restricted in its resolution by either lower one of resolution powers of the image sensor and the LCD panel.
On the other hand, the conventional device with an image input facility, which described in Japanese Laid-open Patent Publication (TOKKAI HEI) No. 4-251824, is constructed on the base of a two-terminal element matrix type LCD panel. This can not be adapted to a TFT-type LCD panel or a simple matrix type LCD panel.
Non-linear resistive elements used for displaying and inputting an original image can not transmit light, so an increase of an area of these elements may darken the display image whilst a decrease of an area of the elements decreases a light-receiving surface area, causing a problem not to obtain large signals of high signal to noise ratio.
The device can optically write an original image into the LCD panel to directly display the image thereon. However, the resolution of the displayed image is restricted by the density of the non-linear resistive elements. In other words, it is limited to the resolution of the LCD panel.
The discrete arrangement of the small-surface non-linear resistive elements may cause a decreased density of input image sampling resulting in increasing an error of the image signal.